A semiconductor element, such as a switching element or a diode, is used in a circuit such as a switching power supply or an inverter. The semiconductor element requires a high breakdown voltage and low on-resistance. The relationship between the breakdown voltage and the on-resistance is a trade-off relationship which is determined by semiconductor material.
With the progress of technical development, the on-resistance of a semiconductor element is reduced to the limit of silicon which is major semiconductor material in use. It is necessary to change the semiconductor material in order to further improve the breakdown voltage or to further reduce the on-resistance.
A GaN-based semiconductor, such as gallium nitride (GaN) or aluminum gallium nitride (AlGaN), has a higher bandgap than silicon. When the GaN-based semiconductor is used as a switching semiconductor material, it is possible to improve the trade-off relationship determined by the semiconductor material and to significantly increase the breakdown voltage or to significantly reduce the on-resistance.
However, for example, a switching element using the GaN-based semiconductor has the problem of “current collapse” in which, when a high drain voltage is applied, on-resistance increases.